There i was looking for dust on the sensor on my D90 and noticed this round lighter circle in the centre of the image.
I thought the sensor was damaged but its only a problem with the 50mm.
Here's a couple of pics. Might be a bit hard too see so ive changed the levels on the second pic so it stands out. These are centre crops of the images.
Camera was pointed at the sky, sun was behind me.
Its noticeable from f16, at f22 its worst.
Any ideas?

Hmm, do you see this only in this one shot or do you have other examples?
What do you see when you look through the lens? Anything inside?
Perhaps that is an UV-hotspot of that lens, that is mostly visible when shooting against clear blue sky???

Thomas, i took about 5 pictures in row and they all had this effect. This picture was the worst of those, or where it was most evident.
I was really only looking for dust, which i found plenty of as well...
Lens looks completely clear, i tried with the UV filter removed as well with the same result.
Also tried taking a picture of the wall inside (with a flash) which also showed the issue.

Trevor, lens is very clean though i did clean it anyway thinking there must be something on the glass..

Can you reproduce the effect with your 50/1.8D, Hilary?
Btw the spot is slightly to the left and above center (and I assume that the image is not cropped).
One question: does the diameter change noticeably (+40%) when using f16 instead of f22? Or does the edge only become a bit blurred - and contrast lower? If it's the first that hole might simply be a ghost-image of the aperture.
Other explanation could be a defect in coating.
Oh well, I'm only guessing here...

HilaryC
Interesting theory, that's not something i really know about.
Ive got an older Tokina 28mm manual prime that i tried and that was fine.
Also went through all my other lenses at this point to see if it was camera, but only the Nikon 50mm did this.

Thomas
The image is a crop from the centre of the image, i wasn't very careful about centring it so the hole is actually in the centre, sorry that's a bit misleading...
Diameter seems to stay the same between f16~f22, just gets more defined.
Not sure about the contrast changing as ive only really pointed it at the sky, i should take some more test shots and see what happens.
Something else i noticed was when i took a picture of inside the room there was an angle poise lamp on. As i rotated the focus ring i got a diffraction grating effect from the lamp through the viewfinder that rotated when focusing. Very strange, i think i need to get the lens back on the camera and look at all this again..
I had a bit of a read up on the UV hotspot of lenses, but only read about the infra red side as that's what the web pages were really about.

Hi HilaryC
I guess that's great news and not at the same time!!
Thanks for taking time out to test it.
I must admit i wouldn't normally have it set much above f8~f11, just had it set higher looking for the dust.

dubaiphil
That will be happening as soon as the cleaning kit turns up, my sensor is filthy

Many thanks for all your help, just need to find someone with the Tokina 11-16mm to test what their aperture does when zooming...

Reflections then, i think i can maybe live with that.
We tried the 50mm D on a Sony A390 with a conversion adapter, it does a similar thing though not such a defined edge, it just gets lighter in the centre.
Thanks for all your input!